text: GUTs predict definite values for the Weinberg angle. This is because GUTs require the SU(2) and U(1) couplings to unify at some energy, and the Weinberg angle is defined in terms of the ratio of these couplings. For example SU(5) apparently predicts sin^2 θ_W = 3/8. This is then modified by renormalization group running between the GUT scale and the scale at which we can do experiments. Check out the last chapter of [Srednicki's textbook](_URL_0_).